Hi all,

I'm new to the frame builder forum, and in fact I'm new to building bikes in general, so go easy on me please.

A little background before I get into the questions.

I'm a late 30-something former IT guy who is in the middle of a career change to welding. I'm currently 7 months into the welding program at a local CC. I'm not the biggest bicycle fan out there, but I've always had an appreciation for their design. I'm particularly interested in recumbent bikes, custom (unique) builds and utility bikes, and older bike frame designs from the early 1900's. My understanding of the inner workings of bikes, design, and components are rudimentary at best, but I'm learning more each day.

In September I had started work on a steel square tube recumbent bike using the TIG (GTAW) process and some plans I got from Atomic Zombie. The bike should be complete next week. I deviated from the plans mid stream and changed the designs and bike's geometry. It's not scientific, but it was fun building something I would eventually ride even if only for a short time until I finish my next bike. I'll post some photos next weekend when it's complete.

I have since sold my TIG welder though, because I needed a MIG welder for other jobs I was working on. I'm aware TIG is the better process for this kind of work, however it simply wasn't an option, so I have to work with what I have.

I currently own a Millermatic 211 which will do between 24 ga. and 3/8" steel, using wire sizes of .024 - .045. I also have the Spoolmate 100 spoogun for aluminum work (.030 wire), which will do between 18 ga. to 1/4". I've got two tanks of gas, pure Argon and C25.

I'm looking to start my second recumbent using round 4130 tubing from Aircraft Spruce (which I learned about from this forum).

So my questions are:

1) What wall thickness would you recommend for building 4130 recumbent bikes? Keep in mind the lengths of the tubing for different parts of the bike's geometry will be quite long compared to a diamond frame upright bike.

2) Having never welded 4130 specifically, I'm assuming mild filler wire and C25 will work but I'd feel silly if I didn't ask. Is this a good combination for welding 4130, or would I require wire with a higher tensile strength and possibly a different gas mixture?

3) Can I weld the 4130 tubing to a standard carbon steel bottom bracket, such as this one: http://www.paragonmachineworks.com/s...-10464059.aspx

The last few questions are about Head Tubes, and perhaps this should go in a different thread, but if I wanted to order tubing to use for a custom head tube for threadless forks, what size (ID) would I order? If this isn't a simple answer, I'd be open to reading anything you guys would care to link me to on the subject. I've read a bunch of general info about head tubes, and sized of the forks, but the tube itself seems to be eluding me.

Thanks a ton for any replies on these questions!